This invention relates to separating a molten metal such as aluminum or aluminum alloy from suspended fine particles typically comprising a buoyant phase such as a buoyant liquid salt phase and to the use of such in combination with processes for purifying molten aluminum which produce a salt reaction product.
Molten metal such as aluminum, including alloys containing over 50% aluminum, has been treated with salt to remove impurities or with a gas which reacts with certain impurities to produce a salt reaction product, typically liquid salt, or combinations of liquid salt with solids and gases. Processes of this type are described in U.S. Pat. Nos. 3,767,382, 3,849,119 and 3,839,019, all of which are incorporated herein by reference. Each of these processes includes some provision for settling such as separation chambers to separate treated molten aluminum from salt, either added in treating the molten aluminum or formed as a reaction product in treating the molten aluminum. However, as such systems are pressed for increased productivity, difficulties can be encountered in separating the suspended salt-bearing and other particles from the molten aluminum. If the particles are not adequately separated and are carried by the molten aluminum to the casting station, such can result in surface and subsurface defects in the cast ingot.
The particles referred to are in the micron size range, typically from less than 1 micron up to 40 or possibly occasionally 50 microns in size (1 micron=10.sup.-4 cm). The particles can comprise liquid salt droplets or solid particles such as solid oxide particles or solid salt particles attached or encapsulated in liquid salt. The particles typically are lighter or heavier than the molten metal and would be gravity separable but remain entrained or suspended largely because of the surface effects caused by their small size. Other than for their small size, the buoyant particles would rise to the surface for removal by skimming or similar operations and the heavier particles would settle downwardly out of the molten metal. However, efforts to date to remove the particles directly have not been entirely satisfactory. For instance, movement through a filter bed of the type shown in U.S. Pat. No. 3,039,864 can result in premature plugging of the filter bed or, in some cases, passage of liquid particles to the casting operation. The salt particles typically result in oxide patches on continuously cast ingot which can give rise to problems in rolling the ingot into plate or sheet products.